Imaging system with a prism having adjustable number of reflections

ABSTRACT

The invention relates to a small-size and light-weight optical system with an adjustable focal length. The invention relates to a method and system comprising an image plane on which there is formed an image of the target observed by the optical system, a first optical component with an adjustable focal length, and a prism with an optical path that is adjustable to correspond to the focal length adjusted for the first optical component. By means of the optical system according to the invention, there is formed an image of the target under observation, located at the distance of the focal length of the first optical component, on the image plane so that the first optical component, the prism and the image plane remain permanently in place in relation to each other.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is for entry into the U.S. national phase under §371for International Application No. PCT/FI2004/050095 having aninternational filing date of Jun. 17, 2004, and from which priority isclaimed under all applicable sections of Title 35 of the United StatesCode including, but not limited to, Sections 120, 363 and 365(c), andwhich in turn claims priority to Finnish patent application 20035097filed on Jun. 17, 2003.

FIELD OF THE INVENTION

The present invention relates to a small-size and light-weight opticalsystem with an adjustable focal length.

BACKGROUND OF THE INVENTION

Optical systems are typically compiled of several optical elements. Inoptical elements and in systems compiled thereof, there always occurimaging errors. In optical planning, the aim is always the best possibleresult from the point of view of the end application. A general aim isthat an object point under observation could be imaged as an imagepoint. For instance in a camera and other optical systems designed forimage reproduction, it is necessary to take into account, in addition tothe features of a possible display device, also the resolution of thehuman eye, when the required image accuracy and pixel size are beingiterated. In general, resolution means the capacity of a system todistinguish details. Resolution is typically expressed as the shortestangular distance between two points that are only just distinguished astwo separate points. The resolution of a healthy human eye in favourableconditions is not more than one arc minute.

Typically optical imaging errors are corrected or compensated bysuitable combination of various lenses. A lens or a lens combinationwith a positive focal length is generally called a positive lens.Positive lenses that collect rays are for instance a convex-convex or apiano-convex lens. Respectively lenses that have a negative focallength, i.e. dispersing, concave lenses or lens combinations, aregenerally called negative lenses. When correcting imaging errors, thereis chosen a suitable glass quality, lens thickness and shape, thelocation of the apertures and the lens coating. Typically when planningan optical system, there is searched an appropriate balance between thedifferent imaging errors, in order to achieve a sufficiently good resultin each case.

Often the image quality is better, the larger the number of lenses thatis used for producing the image. For instance from the point of imagereproduction, an ideal optical system could comprise 20 lenses, becausewhen correctly combined, different glass and lens types mutuallycompensate their imaging errors. However, if the object is to realizelight-weight optics in a small space, the number of lenses must be keptremarkably smaller, which means that the image quality requirements mustbe cut down. It can be roughly maintained that the performance of anoptical system is always better, the larger the number of lenses that itis built of. However, when planning the system, it must also be takeninto account that about 5% of light is reflected back from each lens.Apart from the fact that in multi-lens systems luminosity is decreased,systems containing several elements are heavy and expensive. Even if agood image quality is achieved, the size, weight and expenses restrictthe applicability and usage of this kind of optics.

A known device where optics are generally used is a camera. A prior artoptical solution designed for a small camera is disclosed in thepublication U.S. Pat. No. 6,342,975. In the lens system, the first andsecond lens groups are positive, and the third lens group Is negative.The first lens group includes a negative and a positive lens. The secondlens group includes an adjustable aperture and a negative and a positivelens. The third lens group includes a positive and a negative lens. Whenthe optics are zoomed from a wide-angle close-up view to a telescopictelephoto view, each of the lens groups moves towards the target end,i.e. towards the first lens group. In addition, the distance between thefirst and second lens group grows, and the distance between the secondand third lens group is shortened. This arrangement, however, contains alarge number of single optical elements, which makes the opticalarrangement expensive. Moreover, when the number of lenses increases,the optical system requires more space, and it becomes heavier. Themutual positioning of the optical elements and the realization of theirmechanical mobility as well as their control require precision.

Another simple lens system suited in a small camera is introduced in thepublication U.S. Pat. No. 6,040,949. The arrangement aims in that boththe camera and its optical system can be realized in a small size, assmall as possible. The optical system includes a lens system, and thethree lens groups thereof are illustrated in FIG. 1 representing theprior art. The first lens group 101 in FIG. 1 constitutes aplano-concave lens, and it has a negative refractive power. The secondlens group 103 in FIG. 1 constitutes a convex-convex lens, and it has apositive refractive power. The third lens group 104 in FIG. 1constitutes combined convex-convex and plano-concave lenses, and therefractive power of the third lens group 104 is positive. The targetunder observation that should be pictured, illustrated in FIG. 1, islocated on the left side 105 of the lens group 101, and an image of thetarget is formed on the right side 106 of the lens system. The opticalaxis 107 runs through the center point of the lenses. Between the firstlens group 101 and the second lens group 103, there is an adjustableaperture 102. When focusing the image, i.e. when zooming a certain partof the view, the locations of the second lens group 103 and of the thirdlens group 104 are changed along the optical axis. The describedarrangements advantageously include only 4 single lenses.

In the above cited publications, the lens groups are moved along theoptical axis in order to adjust the focal length of the system. Thereciprocal moving of lenses is a very typical way to adjust the opticalproperties of the system, such as focal length, magnification, focusingor angular field. Another known way, particularly used in cameras, toadjust the focal length or the refractive index is to replace the lensby a different type of lens. This can be realized for instance so thatin front of the camera lens, there is installed a disc containingdifferent types of lenses that can be changed in order to picture thechosen target as accurately as possible. The mechanical moving of thelenses or of the disc always requires space, which is minimal in smalldevices. In addition, the mechanical adjustments must be carried outaccurately both in the installation step and when the device is beingused, in order to arrange the optical components precisely at the rightposition in relation to the optical axis and to each other.

A typical prior art optical system suited in small cameras is a systemwith only one focal length, in which case the focal length is notadjusted mechanically at all. In this arrangement, the distance from thetarget to the camera falls within a fairly limited range if a good imagequality should be maintained. This means that the image only has a highquality within a given average range, but a sharp image cannot be madeof targets located at a close or distant range. Consequently thepossibility to adjust the focal length is a desired feature in opticalapplications and in devices utilizing them.

BRIEF SUMMARY OF THE INVENTION

The objective of the invention is to realize an improved, small-size,compact, light-weight optical system with an adjustable focal length.

The objectives are realized by producing a compact optical systemcontaining a first optical component with a focal length that can beadjusted for various distances, and a second optical component, wherethe optical path of the ray that passes therethrough is adjustable, sothat the optical length of the radiation passing through the secondoptical component corresponds to the focal length of the first opticalcomponent. In addition, the objectives are realized so that the opticalcomponents are installed in place in a stationary manner, and thestationary positions of the components remain the same as the opticalfeatures of the components are changed.

The invention is characterized by what is set forth in the independentclaims. Advantageous embodiments of the invention are described in thedependent claims.

According to an advantageous embodiment of the invention, there isrealized a small-size, adjustable optical system provided with an imageplane, on which there is formed an image of the target observed by theoptical system, a first optical component with an adjustable focallength, and a prism, where the optical path of the ray passing throughsaid prism is adjustable to be such that it corresponds to the focallength of the first optical element. According to an advantageousembodiment of the invention, on the image plane there is formed an imageof the observed target located at the focal length of the first opticalelement, so that the first optical component, the prism and the imageplane are installed in a stationary fashion with respect to each other.

The optical elements of an optical system according to an advantageousembodiment of the invention comprise a variable-shape lens and a prismprovided with an air gap. The optical properties of the lens arepreferably adjusted by changing the shape of the lens. According to anadvantageous embodiment of the invention, the lens need not be shiftedor moved from its stationary position in order to adjust the focallength. When the variable-shape lens is thick, it has a short focallength. In that case the target under observation is located near, andthe angle of view is wide. When the lens is thin and has a long focallength, the optics serve as a so-called tele-objective. The target areaunder observation is located fairly far from the lens, and the view istypically narrow. Thus a thin lens with a long focal length operates ina telescopic fashion.

In order to make on the image plane a sharp image of each point of thetarget surface under observation, the rays passing through the lens forma image point on the image plane that is located at the focal lengthfrom the lens. Because the focal length of the lens can according to theinvention be adjusted, it is respectively necessary also to adjust thelength traveled by the radiation that passed through the lens in orderto make on the image plane a sharp image of the target underobservation. According to an advantageous embodiment, the length thatthe ray has passed through the prism, the so-called optical path,corresponds to the focal length of the lens, so that the length passedby the rays from the target plane to the lens center is as long as thelength passed by the rays from the lens center to the image plane.Instead of moving the lens as in the prior art in order to lengthen theoptical path, the distance traveled by the rays is according to anadvantageous embodiment of the invention adjusted by means ofreflections taking place in the prism. In the prism, the Incoming rayproceeds directly forward, without reflecting from the boundarysurfaces, wherefore the optical path traveled by the ray in the prism isshort. When the focal length of the lens grows, the optical pathtraveled by the ray in the prism can be extended, so that multiplereflections are created in the prism before the ray meets the imageplane. Thus the optical path traveled by the radiation in the prism canbe adjusted without mechanically moving or shifting the optical elementfrom its stationary location.

In an advantageous embodiment of the invention, in order to adjust theoptical path traveled by the ray, there is used a prism provided with anair gap. When the lens is thick and the focal length is short, the airgap of the prism is filled with a liquid having a refractive index thatat a certain accuracy is the same as the refractive index of the prismmaterial. Reflections do not take place in the prism, but the radiationproceeds through it onto the image plane in a nearly straightforwardfashion. When the system is used as a tele-objective, the lens is thinand its focal length is long. Now, according to an advantageousembodiment, the prism gap is emptied of the liquid, after which the gapleft in between the prism planes is filled with air. Because therefractive index of air is different from that of the prism material,the rays are reflected at the boundary surface of the prism material andair. In the prism, the reflected rays further meet other boundarysurfaces that can be coated with reflecting films. Consequently severalreflections take place, and as a result, the optical path of the rayentering the prism is extended. By means of the reflections, the opticalpath and thereby the focal length can be increased by means of so-calledPechan prisms by more than three times, and with some other types ofprisms even by nearly six times.

By means of the optical system according to an advantageous embodimentof the invention, it is possible to make sharp images of targets locatedat different distances, i.e. to adjust the distance between the targetand the camera. This adjusting cannot be carried out by thesingle-focal-length optics typically used in small cameras that areinstalled for example in telephone devices. By using the apparatus andmethod according to the advantageous embodiment of the invention, thereare obtained pictures with a better quality from both close and longrange, because the prior art fixed-focal-length optics are only suitedto be used in a limited middle region. According to an advantageousembodiment of the invention, the focal distance is adjustable to beshort for shooting close-range targets, and to be long for representinglong-range targets. Advantageously the angular field in a close-up viewis wide, and a long-range view is shot in a telescopic fashion, with anarrower angular field. In an optical system according to anadvantageous embodiment of the invention, the adjustments between a wideand a narrow angular field, between a close-up view and a long-rangeview are swift. With multilens camera objectives, where the adjustmentis carried out by moving lenses or lens groups, the desired change inthe angular field is achieved much slower than when modifying theoptical properties of the lens by changing its shape and by filling theprism air gap with liquid or by emptying it of the liquid. In addition,an optical system according to an advantageous embodiment of theinvention that produces a good picture quality is light-weight andcompact in comparison with multilens camera objectives, wherefore it issuited even in small devices.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention is described in more detail below with reference to theappended drawings, where

FIG. 1 illustrates an optical system according to the prior art,

FIG. 2 illustrates an optical system according to an advantageousembodiment of the invention, and

FIG. 3 illustrates an optical system according to an advantageousembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 shows an optical system according to an advantageous embodimentof the invention, the first optical component whereof is a thick lens204 with a short focal length, and the second optical component is aprism 201. The prism 201 is provided with a gap 202 that is filled witha filling agent having the same refractive index as the prism. Thus, inthe prism 201 there is achieved for the ray passing therethrough anoptical path corresponding to the focal distance of the lens 204, andthe target located at the focal length distance from the lens 204 isfocused on the image plane 203.

In the embodiment illustrated in FIG. 2, the lens 204 is a collecting,positive, thick lens. The focal length of the lens is fairly short,wherefore the target 200 under observation, which advantageously islocated at the focal length distance, is relatively near the lens. Thearrow 200 illustrates the target surface between the edge ray 206 andthe center ray 205, which thus is half of the real target surface leftbetween two edge rays. The target surface 200, i.e. the view pictured inthis embodiment is wide. In addition, in FIG. 2 there is illustrated animage plane 203, on which there is formed an image of the targetobserved by the optical system. Here the term image plane generallymeans a location or a surface on which there is formed an image of thetarget under observation by means of the optical system according to theinvention. The image plane does not necessarily have to be a plane. In aprism 201 according to an advantageous embodiment of the invention,there is provided a planar gap 202, the width whereof is of the order ofa few micrometers. Advantageously the gap runs through the prism, fromone angle to the opposite angle, forming a plane so that all rayspassing through the prism pass through the gap. In the advantageousembodiment illustrated in FIG. 2, the gap 202 is filled with liquidhaving a refractive index that with a certain accuracy is the same asthe refractive index of the prism material. In that case the refractiveproperties of the prism correspond to those of a uniform prism without agap. The radiation, which generally is light radiation, penetrates saidprism 201 provided with a gap in the same fashion as it would penetratea homogeneous prism.

The passage of the rays emitted from the object under observation isillustrated by following the passage of the center ray 205 and of theedge ray 206. The passage of the center ray 205 from the target plane200 through the optical system onto the image plane 203 is depicted by auniform line. The edge ray 206 is depicted by a dotted line. The centerray 205 enters the optical system via its center point, along theoptical axis. In this embodiment, the center ray 205 passes through thecenter point of the lens 204 located on the optical axis. The positive,convex lens 204 collects the rays emitted from the image plane anddirects them further to the prism 201. The distance of the image plane203 from the lens is determined according to the focal length of thelens. Advantageously the target surface or plane under observation islocated at the focal length distance from the lens. In the prism 201,the rays are refracted directly on the image plane 203, where there isformed an image point corresponding to that point of the target plane200 from which the ray was emitted. The edge ray 206 hits the lens 204in an inclined way. The lens collects the rays, and they are refractedin the prism 201, so that the edge ray of the target plane 200 ispictured at the opposite edge of the image plane 203. The optical pathtraveled by the rays in the prism 201 corresponds to the focal length ofthe lens 204. Of one point of the target plane 200, there isadvantageously formed one image point on the image plane 203.

In the embodiment illustrated in FIG. 2, the target plane 200, i.e. theview reproduced by the optical system, is wide. In other words, thesystem has a wide angular field. A wide angular field is typicallyachieved when the focal length of the optical system is relativelyshort.

FIG. 3 illustrates an optical system according to another advantageousembodiment of the invention, which system contains corresponding opticalcomponents as the embodiment illustrated in FIG. 2. In the advantageousembodiments of the invention the first optical component is avariable-shape lens, in which case its optical properties, hereadvantageously the focal length, can be adjusted by changing the shapeof the lens, typically its curvature. The first optical component mayalso be a lens combination composed of several single lens elements.Advantageously at least one lens elements is a variable-shape lens. Theemployed second optical component is a prism, and the optical path ofthe rays passing through said prism can be adjusted so that the opticalpath traveled by the rays in the prism corresponds to the adjustablefocal length of the lens in each case. In the prism, the optical path ofthe rays passing therethrough is adjusted advantageously by adjusting inthe prism gap the total reflections taking place in the prism.Advantageously the optical elements of the invention are not shifted,but they have stationary positions, and their optical properties can bechanged without moving them.

In the embodiment of FIG. 3, the optical system is used for shooting along-range view. The first optical component is a thin, variable-shapelens 304 with a long focal length. The second optical component is aprism 301 provided with an air-filled gap 302. The refractive index ofair is different from that of the prism 301, and the optical pathtraveled by the ray in the prism 301 corresponds to the focal length ofthe lens. The image of the target under observation is formed on theplane 303.

In FIG. 3, the focal length of the optical system is long, which meansthat the angular field is small and the view, i.e. the target areapictured on the image plane is fairly limited. The target area 300formed in between the center ray 305 and the edge ray 306 illustrated inthe drawing is half of the real target area formed in between the edgerays. Typically the arrangement according to the described embodiment isused as a tele-objective, when imaging distant targets. The view more orless corresponds to that of a telescope: distant targets on a relativelysmall area are focused well on the image plane. In this embodiment,there is typically used a thin lens 304 with a long focal length. A thinlens here means a lens where the thickness is small in comparison withthe radius of the spherical surface thereof. A thin lens is suited to beused when the incoming rays are nearly parallel with the optical axis,i.e. they have a small angle of incidence. When the angle of incidenceis small, as in FIG. 3, also the rays enter the optical system at asmall angle of incidence.

In the embodiment illustrated in FIG. 3, the gap 302 of the prism 301 isemptied of the filling agent, i.e. it is filled with air. In theembodiment illustrated in FIG. 3, the passage of the center ray 305 fromthe target plane 300 through the optical system onto the image plane 303is depicted by a uniform line. The edge ray 306 is depicted by a dottedline. The center ray 305 passes along the optical axis running via thecenter point of the system to the prism, where it is reflected from theedge of the air gap 302 to the opposite edge of the prism 301. From theedge of the prism 301, the refracted ray meets the air gap 302 at asufficiently large angle in order to penetrate the air gap 302, and thenthe ray proceeds to that edge of the prism that faces the image plane303. The angle of incidence to the edge facing the image plane 303 is,however, so small that the ray is again reflected onto the adjacent sideof the prism 301 and further to the edge of the air gap 302, from wherethe ray is refracted onto the image plane 303, thus forming an imagepoint there.

The edge ray 306 passes to the first component of the optical system,which here is the lens 304. The positive lens 304 collects the rays anddirects then onto the prism 301 at a given angle of incidence. The edgeray 306 is refracted from the edge of the air gap 302 of the prism 301onto the opposite side of the prism 301. From the side of the prism 301,the refracted ray enters the air gap 302 in a nearly perpendicularposition, so that a total reflection does not take place anymore, butthe ray penetrates the air gap 302 along a straight line to that side ofthe prism 301 that faces the image plane 303. From here, the edge ray306 Is reflected to the adjacent side of the prism 301, whereafter itproceeds via that edge of the prism 301 that faces the air gap 302 andis reflected from there onto the image plane 303, where it forms animage point.

In this advantageous embodiment, the light entering the prism 301 isrefracted several times in the prism 301 before it reaches the imageplane 303. The optical path traveled by the light in the prism 301 isadvantageously even 3.8-fold in comparison with the path traveled bylight in a homogeneous prism. Generally the optical path traveled byrays in the prism 301 is extended, when total reflections are created onthe boundary surface of the gap and the prism. Total reflections arecreated when the gap filling agent is chosen to have a suitablerefractive index, and the rays enter it at a given angle. Typically theemployed totally reflecting gap filling agent is air, which has adifferent refractive index than the prism material.

From the point of view of the invention it is advantageous that theoptical properties of the lens can be changed without changing the lensitself, or without moving it along the optical axis or in relation toother optical elements or image/target planes. For instance theproperties of a variable-shape lens are changed by changing thecurvature of the refracting lens surface. Advantageously variable-shapelenses are elastic, so that their diameters can be extended andshortened. From the point of view of the invention, the shape of therefracting lens surface is not essential, and neither is the grinding ofthe lens. In the optical system according to the invention, there can beapplied both a spherical lens as well as for example a ground Fresnelllens. According to an advantageous embodiment, the lens material can beany suitable, permeable and elastic material, such as silicon, othersynthetic polymer, rubber, polyethylene or polypropylene. In particular,the lens must be permeable to such wavelengths that are desired to beobserved by the system according to the invention, i.e. typically thewavelengths of visible light. Obviously an essential feature is thepermeability of the material, which requires that the material absorbsincoming radiation as little as possible, and that the scattering orbreaking of incoming rays is minimal.

In an advantageous embodiment of the invention, the employed lens mayalso be a lens combination, where the lenses forming the combination arepreferably permanently connected to each other in a way known as such.The lens may for instance comprise a thin, permeable shell that isfilled with an agent, which agent is for example liquid, gas or agel-like substance. According to an advantageous embodiment of theinvention it is essential that the employed lens is highly permeable andelastic.

A change of a few percentages in the diameter changes the refractivecapacity and focal length of the lens. Variable-shape lenses generallyhave relatively short diameters. The changing of the lens shape can berealized for instance so that the elastic lens is evenly stretched atthe edges. The lens is thickest in a so-called idle state, and itsdiameter can be evenly stretched; as a consequence, the lens becomesthinner, and its optical properties are changed. One way for realizingthe changing of the lens diameter Is to install extremely small-sizemicroelements evenly on the circumference of the lens. On the basis ofcontrol signals obtained from the control unit, each element causes amechanical compressive or tensile effect in order to move the lenscircumference inwards or outwards from the center point of the lens. Theelements according to another advantageous embodiment are annular-shapedelectrodes with various sizes, and they are placed concentrically withrespect to the center point of the lens. The controlling of thesemicroelements can be realized for example electrically, by switchingpower in them, and by then inducing an electric field in the lens.

According to another advantageous embodiment, the lens may consist oftwo mutually non-soluble ingredients. For instance in the publicationU.S. Pat. No. 6,369,954 B1, there is described a variable-shape lenscomposed of a first conductive liquid and of second insoluble liquidthat is separated at the other edge of the lens, which second liquid isneither mixed nor dissolved in the first conductive liquid, being forinstance clear oil. The employed liquids have different refractiveindexes. In the lens, there are coupled electrodes that are connectedvia a switch to a power supply or to earth. The shape of the boundarysurface of the liquids constituting the lens is changed by means of theelectric field induced over the lens. The shape of the boundary surfacecan be adjusted for example by conducting the power only to certainelectrodes coupled in the lens. Typically, when the power value isalternated between zero and the maximum value, the boundary surface ofthe substances changes its shape between its two extreme positions. Thiskind of method is called electrowetting.

The lens according to a third advantageous embodiment is a crystallinedoubly refracting lens. The lens consists of doubly refracting crystalswith a horizontal refractive index that is different than the verticalrefractive index. The crystals in the lens are arranged in a givenorder, for instance all horizontally, evenly and equidistantly. Thecrystals are typically turned so that there is induced an electric fieldover the crystalline lens. By means of the electric field, thehorizontal crystals are arranged according to the electric field, forexample in a given position, such as at a given angle or in a verticalposition, in which case the refractive index of the crystalline lens ischanged. Crystalline lenses and the controlling of crystals is describedfor instance in the publication JP2001272646.

According to an advantageous embodiment of the invention, the opticalproperties of the prism are modified by changing the optical pathtraveled by the entering rays in the prism. Advantageously the appliedprism is provided with a gap, or the prism is formed of two prisms,where a small gap is formed between the mutually facing sides. The prismaccording to the advantageous embodiment, provided with a gap, is forexample a known Pechan prism. By changing the gap filling agents,according to the refractive index of the filling agent, the enteringrays can be controlled to proceed in the prism in a desired way. Forinstance when it is desirable that the entering ray travels in the prisma length that is as short as possible, the gap filling agent is chosenso that the ray proceeds without refracting and without being reflected,directly through the gap. Now the filling agent is chosen so that itsrefractive index is the same, or at least with a certain accuracy thesame as the refractive index of the prism material. When the opticalpath should be extended, it is attempted to achieve total reflectionfrom the gap edge back into the original segment of the prism. By meansof the created total reflections and other reflections, the optical pathof the light passing through the prism can be adjusted to be overthreefold. The optical path of light, or more generally of the ray,traveled in the prism is according to an advantageous embodiment of theinvention adjusted in the same proportion and in a corresponding fashionas the focal length of the first optical component. The target surfacepictured by an optical arrangement according to the advantageousembodiment can be defined even at a distance that is 3.8-fold from theoptics, so that a sharp image of the target is obtained on the imageplane without having to mechanically move the optical elements, such asthe prism, the variable-shape lens or the image plane.

The gap in a prism according to an advantageous embodiment of theinvention is very thin. The filling of the gap with a given liquid, andthe removing of the liquid from the gap, can be realized in manydifferent ways. According to an advantageous embodiment, in the fillingand re-emptying of the gap, there is made use of the capillaryphenomenon. Typically the filling of the gap with liquid and theemptying of the gap of the filling agent are controlled by means of anelectric field. The prism filling and emptying can also be controlled bymeans of electrowetting that was mentioned in previous, in thedescription of the modification of the lens shape. The guiding of theliquid in the narrow gap is described in more detail for example in thepublications US 2,002,080,920 and U.S. Pat. No. 4,701,021.

The adjusting of the focal length of the optical system according to theadvantageous embodiment of the invention by modifying the shape of thelens and by changing the filling agent in the prism gap is faster thanthe adjustments made in the prior art, which always require the movingof the optical elements or optical components made of several elements.In addition, the arrangement according to the invention where the focallength is adjusted fits well within a small space, wherefore it isparticularly well suited in small devices, for instance mobile phones.The possibility to adjust the focal length improves the quality of theoptics, and light-weight optics are achieved, when the arrangementincludes as few optical components as possible, here advantageously onlytwo. For a man skilled in the art it is obvious that the method andarrangement according to the advantageous embodiment of the inventioncan be technically realized in many different ways, by using variousdifferent components and component combinations, without departing fromthe scope of the invention.

1. An apparatus comprising: an image plane, on which there is formed animage of a target, and a lens permanently positioned relative to saidimage plane, wherein the lens has a mechanically adjustable focallength, and a prism permanently positioned relative to said image planeand said lens, with an optical path that is adjustable so that itcorresponds to the focal length determined for the lens, so that animage of an observed target located at the focal length of the lens isformable on the image plane, wherein the prism is configured to producean adjustable number of reflections along the optical path through theprism.
 2. The apparatus according to claim 1, wherein the lens is avariable-shape lens with a curvature that can be changed in order toadjust the focal length of the lens.
 3. The apparatus according to claim1, wherein the lens is an elastic, variable-shape lens with a diameterthat can be changed in order to adjust the focal length of the lens. 4.The apparatus according to claim 1, wherein the lens is made of twomaterials that are mutually non-soluble, so that the shape of theboundary surface of said materials can be changed in order to adjust thefocal length of the lens.
 5. The apparatus according to claim 1, whereinthe lens contains doubly refracting crystals, the position of saidcrystals being changeable in order to adjust the focal length of thelens.
 6. The apparatus according to claim 1, wherein an electric fieldis to be induced in the lens in order to adjust the optical propertiesof the lens.
 7. The apparatus according to claim 1, wherein the lens iscomposed of several elements, at least one of which is a variable-shapelens.
 8. The apparatus according to claim 1, wherein the prism isprovided with a gap, and that the apparatus comprises means for fillingthe gap by a filling agent and for emptying the gap of the filling agentin order to adjust the optical path in the prism.
 9. The apparatusaccording to claim 8, wherein the gap of the prism is filled with aliquid filling agent with a refractive index that is the same as therefractive index of the prism in order to minimize the optical path inthe prism.
 10. The apparatus according to claim 8, wherein the gap ofthe prism is filled with air as the filling agent in order to maximizetotal reflections and the optical path in the prism.
 11. The apparatusaccording to claim 1, wherein the lens is a thick, variable-shape lenswith a short focal length, and that the prism is provided with a gapfilled with a filling agent that has the same refractive index as theprism in order to achieve an optical path corresponding to the focallength of the variable-shape lens and in order to focus the targetlocated at the focal length on the image plane.
 12. The apparatusaccording to claim 1, wherein the lens is a thin, variable-shape lenswith a long focal length, and that prism is provided with a gap filledwith air that has a different refractive index than the prism in orderto achieve a focal length corresponding to the focal length of thevariable-shape lens and in order to focus the target located at thefocal length on the image plane.
 13. The apparatus according to claim 1,wherein it is installed in a portable mobile phone.
 14. A methodcomprising: adjusting mechanically a focal length of a lens tocorrespond to a distance between a target under observation and saidlens, permanently positioning the lens relative to an image plane,permanently positioning a prism relative to the lens and to the imageplane, adjusting an optical path of the prism to be such that itcorresponds to the adjusted focal length of the lens by producing anadjustable number of reflections along the optical through the prism,and forming an image of the target on the image plane.
 15. The methodaccording to claim 14, wherein said lens is a variable shape lens andthe curvature of the variable-shape lens is changed in order to adjustthe focal length of the lens to correspond to the distance of the targetunder observation.
 16. The method according to claim 14, wherein saidlens is an elastic variable-shape lens and the diameter of the elasticvariable-shape lens is changed in order to adjust the focal length ofthe lens to correspond to the distance of the target under observation.17. The method according to claim 14, wherein the shape of a boundarysurface of substances of the lens is formed of two mutually non-solublesubstances and is changed in order to adjust the focal length of thelens to correspond to the distance of the target.
 18. The methodaccording to claim 14, wherein the position of crystals contained in thelens contains doubly refracting crystals and is changed in order toadjust the focal length of the lens to correspond to the distance of thetarget under observation.
 19. The method according to claim 14, whereinan electric field is induced in the lens in order to adjust the opticalproperties of the lens.
 20. The method according to claim 14, whereinthe optical path of the prism is adjusted by a gap provided in theprism, so that the gap is in turn filled with a filling agent and inturn emptied of the filling agent.
 21. The method according to claim 14,wherein the focal length of the lens is adjusted to be short, and that agap of the prism is filled with a filling agent with a refractive indexthat is the same as the refractive index of the prism in order toachieve in the prism a focal path that corresponds to the focal lengthof the lens and to focus the target located at the focal length on theimage plane.
 22. The method according to claim 14, wherein the focallength of the lens is adjusted to be long, and that a gap of the prismis filled with air that has a different refractive index than the prismin order to achieve, by means of total reflections in the prism, anoptical path that corresponds to the focal length of the lens and inorder to focus the target located at the focal length on the imageplane.
 23. A method for manufacturing an optical system comprising:installing an image plane, on which an image of a target is formed,installing a lens with a focal length that is mechanically adjustable tocorrespond to the distance of the target under observation, andpermanently positioning the lens relative to said image plane,installing a prism with an optical path that is adjustable to correspondto the adjusted focal length of the lens, the prism configured toproduce an adjustable number of reflections along the optical paththrough the prism, and permanently positioning the prism relative to thelens and to the image plane.
 24. A mobile phone comprising an adjustableoptical system, the adjustable optical system comprising: an imageplane, on which there is formed an image of a target observed by theoptical system, and a lens permanently positioned relative to said imageplane, with mechanically adjustable focal length, a prism permanentlypositioned relative to said image plane and said lens with an opticalpath that is adjustable so that it corresponds to the focal lengthdetermined for the lens by configuring the prism to produce anadjustable number of reflections along the optical path through theprism, so that an image of an observed target is formable on the imageplane.
 25. The mobile phone according to claim 24, wherein the lens is avariable-shape lens with a curvature that can be changed in order toadjust the focal length of the lens.
 26. The mobile phone according toclaim 24, wherein the lens is an elastic, variable-shape lens with adiameter that can be changed in order to adjust the focal length of thelens.
 27. The mobile phone according to claim 24, wherein the lens madeof two materials that are mutually non-soluble, so that the shape of theboundary surface of said materials can be changed in order to adjust thefocal length of the lens.
 28. The mobile phone according to claim 24,wherein the lens contains doubly retracting crystals, the position ofsaid crystals being changeable in order to adjust the focal length ofthe lens.
 29. The mobile phone according to claim 24, wherein anelectric field is to be induced in the lens in order to adjust theoptical properties of the lens.
 30. The mobile phone according to claim24, wherein the lens is composed of several elements, at least one ofwhich is a variable-shape lens.
 31. The mobile phone according to claim24, wherein the prism is provided with a gap, and that the systemcomprises means for filling the gap by a filling agent and for emptyingthe gap of the filling agent in order to adjust the optical path in theprism.
 32. The mobile phone according to claim 31, wherein the gap ofthe prism is filled with a liquid filling agent with a refractive indexthat is the same as the refractive index of the prism in order tominimize the optical path in the prism.
 33. The mobile phone accordingto claim 31, wherein the gap of the prism is filled with air as thefilling agent in order to maximize total reflections and the opticalpath in the prism.
 34. The mobile phone according to claim 24, whereinthe lens is a thick, variable-shape lens with a short focal length, andthat the prism is provided with a gap filled with a filling agent thathas the same refractive index as the prism in order to achieve anoptical path corresponding to the focal length of the variable-shapelens and in order to focus the target located at the focal length on theimage plane.
 35. The mobile phone according to claim 24, wherein thelens is a thin, variable-shape lens with a long focal length, and thatprism is provided with a gap filled with air that has a differentrefractive index than the prism in order to achieve a focal lengthcorresponding to the focal length of the variable-shape lens and inorder to focus the target located at the focal length on the imageplane.
 36. An apparatus comprising: an adjustable optical system, theadjustable optical system comprising: means for forming an image of atarget observed by the optical system on an image plane, and means,permanently positioned relative to said image plane, for providing anadjustable focal length, means, permanently positioned relative to saidimage plane and said means for providing an adjustable focal length, forproviding an optical path that is adjustable so that it corresponds tothe focal length determined for the means for providing an adjustablefocal length, so that an image of an observed target is formable on theimage plane.